I walked all the way around the Xi'An city walls. It's certainly a long walk, but away from the most touristy sections it's quite a peaceful walk. If you haven't got 4-5 hours a bike is a good choice to make a circumnavigation. Rent a cycle (mountain bike or tandem) at various points and drop it off at any other station. There are a few crowded spots where bikes aren't allowed but you can drop one off on side and pick up another the same on the other side.